In many industries, from time to time, it is necessary to apply coatings to hot surfaces. The coatings function in the nature of a parting agent between the surface to which they have been applied and some other material which otherwise would come in contact with that surface and adversely interact with it.
Specifically, for example, in the development of piston casting equipment of the type disclosed in the Louis Badone et al application Ser. No. 188,056 now U.S. Pat. No. 3,977,460 and in the R. J. Fulton et al application Ser. No. 293,712 now U.S. Pat. No. 3,979,033, there is the problem of preventing molten aluminum from sticking to the inside of a ladle used to pour the aluminum in molding the pistons. There are several undesirable aspects to the problem.
Firstly, if aluminum in the form of aluminum and aluminum oxide skins remain on the inside of the ladle when additional molten aluminum is added to the ladle, this material tends to be included in the pour as the molten aluminum is poured and causes undesirable defects in the pistons produced.
Secondly, with the stream of metal which fills the ladle always hitting the same spot due to the automatic ladle filling arrangement used with the above referenced equipment, erosion of the inside of the ladle takes place at that spot so that mechanical adhesion of aluminum and aluminum oxide skins to the ladle results.
Thirdly, any of the deposits which adhere to the inside of the ladle can deflect the metal stream so that in extreme cases, metal does not enter the mold but pours over the molding equipment.
It is accordingly desirable to have a ladle in which the oxide skin, while it is still hot and flexible, can be removed by turning the ladle over so that the skin drops out of the ladle before the ladle is refilled with molten aluminum. Various ladle materials have been tried, but none have been found which have the desired non-sticking characteristics. The well established practice of coating the ladle with various state-of-the art parting agents has also been tried. For example, the hand application of mixtures of grease and graphite have been found to generally give the desired behavior but have the disadvantageous that one necessarily relies upon many people to carry out the coatings and the method of application has thus been variable. Additionally, the grease in contact with the hot ladle surface bursts into flame and produces large amounts of smoke and unsatisfactory working conditions. Added to this are odors and excessive heat exposure to the operator.
It is therefore obvious that some other kind of coating is required, preferably an automatically applied coating, both from a consistency of application and an environmental control point of view.
For a time, the use of colloidal graphite dispersions, which have been used for a long time in metal working processes as parting agents, looked attractive. Colloidal graphite suspended in a variety of carrier liquids is commercially available and the idea was to find a colloidal graphite dispersion which could be sprayed into a ladle after each pour therefrom, before the refilling of the ladle, so that the aluminum and aluminum oxide skin would drop out of the ladle cleanly prior to its being refilled.
Spraying is preferred for application because it is fast and more uniform. The time available for coating in an automatic set-up of the tupe referred to above is very short, amounting to only two or three seconds between the return of the ladle to the filling position after pouring and the ladle being filled.
Initially colloidal graphite suspended in three materials respectively; namely, water, isopropyl alcohol and mineral spirits (kerosine type material) was tested.
Graphite in water was unsatisfactory since a coating could not be established at the temperature that the ladle was operating. The spray appeared to go into the ladle and ricochet out again with very little material being deposited on the ladle surface. The little material deposited did not give any marked parting action since very little graphite was actually deposited.
Graphite in isopropyl alcohol gave very little graphite material adhering to the ladle surface but gave somewhat better separation of the aluminum and aluminum oxide skin residue from the ladle surface. However, the alcohol showed a great tendency to ignite during spraying and also dried in the spray heads and host lines under the hot conditions existing in the foundry area and this gave a large amount of unreliability. Generally, the use of large amounts of alcohol as a carrier for the graphite is considered to be unsatisfactory and dangerous in a foundry.
The third material tried was colloidal graphite dispersed in mineral spirits or kerosine. A commercially available variety was obtained from Acheson Colloids of Brantford, Ontario, Canada, identified by them as Acheson Dispersion No. 2404. When diluted further with kerosine and sprayed into a ladle, much more graphite was deposited, although a substantial amount of material still bounced back out of the hot ladle to cover surrounding areas. The separation of the aluminum and aluminum oxide skin from the ladles was much better with this material and in general, ignition of the kerosine type solvent did not occur. Although, from time to time when the spraying conditions were unsatisfactory due to spray heads going out of adjustment, large sheets of flame would be produced during the coating process. In addition, the vapor from the mineral spirit carrier made for unsatisfactory working conditions in the area. This type of material was used in actual production in a foundry for a short period, but the smell and irritation to eyes caused by the vapor caused the abandonment of this material as a practical coating.